\documentclass[tikz, convert={density=3000, outext=.png}]{standalone}
%\documentclass[convert={density=3000, outext=.png}, multi=true]{standalone}

% ===== packages =====
\usepackage{amsmath}
\usepackage{amssymb}
\usepackage{ss2-figures}

% ===== environment for non-tikz pages =====
\newenvironment{image}{\ignorespaces}{\unskip}
\standaloneenv{image}